The method of press molding (called the “precision press molding method”, “precision mold pressing method” etc.) a molding material (referred to as a “press-molding glass material” or “glass preform”, hereinafter) by means of an upper mold and lower mold having opposed molding surfaces is a known method of manufacturing optical elements such as glass lenses. In this method, a piece of optical glass that has been solidified into a prescribed shape from a molten state, or a piece of optical glass that has been polished into a prescribed shape, is placed within a pressing mold and hot press molded to obtain a glass optical element. The precision press molding method makes it possible to inexpensively obtain high-performance lenses because the use of a precisely machined pressing mold obviates the need for post-processing such as polishing following press molding.
However, in the course of molding the glass optical element by the precision press molding method, the press-molding glass material and the molding surfaces of the mold adhere in a high temperature state. Thus, there are problems in that chemical reactions take place and fusion occurs at their interface, compromising mold release.
A method of solving such problems is proposed in Reference 1 (Japanese Unexamined Patent Publication (KOKAI) Heisei No. 8-217468), which is expressly incorporated herein by reference in its entirety, in the form of the reheat pressing method, for example, which employs a glass material for molding that is obtained by forming a carbon film by the thermal decomposition of a hydrocarbon gas on the surface of a glass preform to prevent fusion of the molds to the glass preform.
Reference 2 (Japanese Unexamined Patent Publication (KOKAI) Heisei No. 8-259241), which is expressly incorporated herein by reference in its entirety, discloses a method of using a glass material for molding obtained by forming a hydrocarbon film 20 nm to 30 nm in thickness on the surface of a glass preform by inducing a high-frequency electrical discharge using methane gas, or drawing out an ion beam using methane gas and hydrogen gas. It is described that using such a glass material for molding makes it possible to achieve a good mold release property, even in the press molding of lens shapes that do not readily lend themselves to mold release.
Reference 3 (Japanese Unexamined Patent Publication (KOKAI) Heisei No. 9-286625) and English language family member U.S. Pat. No. 5,851,252, which are expressly incorporated herein by reference in their entirety, disclose the method of employing a glass material for molding obtained by forming a carbon film of less than 5 nm on the surface after subjecting a glass preform to methane plasma processing to enhance the mold release property between the metal molds and the molded product.
In the methods described in References 1 to 3, a thin carbon-based film is formed on the glass preform. However, the glasses containing components that impart high refractive indexes in the form of W, Ti, Bi, Nb, and the like to achieve the high refractive indexes demanded of glass lenses in recent years tend to react with such a carbon-based thin film in the elevated temperature environment during press molding. This is because these high refractive index-imparting components reduce readily, tending to undergo oxidation reduction reactions due to their ability to assume multiple valences while present as glass components, and in the process of press molding, are thought to cause various interface reactions in the process of deforming while being pressed by the molds.
By contrast, in addition to thin carbon-based films, it is known that silicon oxide films can be formed on glass preforms.
For example, Reference 4 (Japanese Examined Patent Publication (KOKOKU) Heisei No. 2-1779), which is expressly incorporated herein by reference in its entirety, describes achieving a lens having good optical quality by forming an SiO2 film of 50 to 2,000 Angstroms, desirably 100 to 1,000 Angstroms, by vapor deposition or sputtering on a PbO-containing glass or alkali borosilicate glass.
Reference 5 (Japanese Unexamined Patent Publication (KOKAI) Heisei No. 7-118025), which is expressly incorporated herein by reference in its entirety, describes a method of blocking volatile components from a glass material by vapor depositing a 100 Angstrom SiO2 film on the glass material in regions where the gap between the glass material and the lower mold during pressing is equal to or less than 0.1 mm.
Reference 6 (Japanese Unexamined Patent Publication (KOKAI) Heisei No. 8-198631), which is expressly incorporated herein by reference in its entirety, describes a method of forming a surface layer by heat treating a glass material and forming a 5 to 50 nm coating containing 70 to 90 mass percent of SiO2 thereover by vapor deposition or sputtering.